In recent years, as image pickup devices have become more compact, the need for high refractive index, high dispersion lenses has increased. High refractive index, high dispersion optical glasses with bases in the form of the phosphate glasses of the compositions disclosed in Patent Reference 1 (Japanese Unexamined Patent Publication (KOKAI) No. 2006-111499) and Patent Reference 2 (Japanese Unexamined Patent Publication (KOKAI) No. 2007-15904) have been employed as such lens materials.
Ultrahigh refractive index, high dispersion glasses affording refractive indexes exceeding an nd of 2.05 and having an Abbe number of 18.5 or lower are effective for providing high zoom ratio, wide-angle lenses. When classifying the elements necessary for constituting an optical glass of high refractive index from the perspective of the characteristics each element imparts to the glass, it is possible to consider high refractive index, high dispersion elements that impart targeted optical characteristics to the glass, glass network-forming components that promote glass formation but are of low refractive index, and modifying components that enhance the melting property of the glass but are of somewhat low refractive index. Since a large quantity of high refractive index, high dispersion elements such as Bi, Ti, and W must be incorporated as glass components to prepare an ultrahigh refractive index, high dispersion glass, the proportion of network-forming components that promote glass formation and modifying components that enhance the glass melting property is kept relatively low.
As a result, the tendency of the melt comprised of the above elements to crystallize intensifies, the thermal stability of the glass decreases, and the liquidus temperature rises. To prevent devitrification, that is, to prevent crystal precipitation, during the preparation of such a glass, the glass melt molding temperature must be raised, thereby resulting in an extremely low viscosity during molding.
Accordingly, defects such as striae tend to occur during molding, and there are problems in that it becomes difficult to obtain a high-quality glass.
The present invention has as its object to provide an optical glass that solves the above problems, has a high refractive index and high dispersion characteristics in the form of a refractive index nd exceeding 2.05 and an Abbe number vd of 18.5 or lower, and affords viscosity characteristics suited to the preparation of a high-quality glass.